cli: allow overwriting non-scalar with jj config set

Before this patch, it was an error to run `jj config set --user foo
'[1]'` twice. But it's only been broken since the previous commit
because '[1]' was interpreted as a string before then.

cli/src/cli_util.rs

@@ -2194,14 +2194,17 @@ pub fn write_config_value_to_file(
                 ))
             })?;
     }
-    // Error out if overwriting non-scalar value for key (table or array).
+    // Error out if overwriting non-scalar value for key (table or array) with
+    // scalar.
     match target_table.get(last_key_part) {
         None | Some(toml_edit::Item::None) => {}
         Some(toml_edit::Item::Value(val)) if !val.is_array() && !val.is_inline_table() => {}
         _ => {
-            return Err(user_error(format!(
-                "Failed to set {key}: would overwrite entire non-scalar value with scalar"
-            )));
+            if !item.is_array() && !item.is_inline_table() {
+                return Err(user_error(format!(
+                    "Failed to set {key}: would overwrite entire non-scalar value with scalar"
+                )));
+            }
         }
     }
     target_table[last_key_part] = item;

cli/tests/test_config_command.rs

@@ -384,6 +384,16 @@ fn test_config_set_type_mismatch() {
     insta::assert_snapshot!(stderr, @r###"
     Error: Failed to set test-table: would overwrite entire non-scalar value with scalar
     "###);
+
+    // But it's fine to overwrite a non-scalar by a non-scalar
+    test_env.jj_cmd_success(
+        &repo_path,
+        &["config", "set", "--user", "test-table.foo", "[1,2,3]"],
+    );
+    test_env.jj_cmd_success(
+        &repo_path,
+        &["config", "set", "--user", "test-table.foo", "[4,5,6]"],
+    );
 }
 
 #[test]